A method for manifesting early strength is generally classified into two types: one using a normal Portland cement as it is and the other using a normal Portland cement mixed with an early-strength admixture.
In order to manifest a high strength at an early age of an existing normal Portland cement, a high-early-strength cement or an early-strength admixture prepared to have great powder or a great amount of components ensuring early strength is intermixed to give a much higher early strength in comparison to a high-early-strength cement concrete. However, at this time, economic feasibility and durability should be considered.
In other words, when an early-strength admixture is intermixed, costs should be considered since the early-strength admixture is much more expensive than the high-early-strength cement concrete. Also, since the early-strength admixture weakens resistance against freezing-thawing or cracks, long-term public service is deteriorated.
The high-early-strength concrete gives a compressive strength of 21 MPa, which is a usable strength, within 3 days, and early strength is manifested using a high-early-strength cement, an early-strength admixture or an early-strength chemical admixture. An ultrahigh-early-strength concrete manifests a usable strength in 1 day and generally gives an early strength by using an early-strength admixture.
An ultra-rapid-set cement manifests a usable strength 3 hours after being mixed with water and thus is used for various emergent constructions. The ultra-rapid-set cement is classified into an alumina-based ultra-rapid-set cement, an amorphous alumina-based ultra-rapid-set cement and a Hauyne ultra-rapid-set cement depending on minerals giving a rapid-hardening property. The ultra-rapid-set cement is obtained by mixing ultra-rapid-set clinker powder to a normal Portland cement by the content of 20 to 45% depending on its rapid-hardening characteristic.
In Korea, various kinds of ultra-rapid-set cements have been developed and frequently used since 1970, but these cements are not stable for a long term since products after hydration reactions are isolated. In particular, since these cements are weak against calcium chloride, when these cements are exposed to a deicing agent used in winter, durability performance is deteriorated, for example due to severe surface separation.
In addition, in case of the high-early-strength concrete, an initial setting generally appears after 60 minutes, a concrete mixer truck may be carefully used if a transporting distance is not long. However, in case of an ultrahigh-early-strength concrete and a rapid-hardening concrete, an initial setting generally appears within 60 minutes, and thus it is substantially impossible to produce these concretes at a batcher plant and transport them using a concrete mixer truck. Therefore, in this case, a mobile mixer or a small mixer is used, which increases production costs.
Here, the initial setting means a state where a cement paste is still soft but has no fluidity, and the time at this is called an initial setting time. Also, the final setting means a state where the cement paste looks like being coagulated as time passes, and the time at this is called a final setting time.